1. Technical Field
The present invention relates to optical flow meters for sensing the velocity of fluids, including mixtures of gaseous and liquid fractions such as steam, moving in a pipe.
2. Background
The need for measurement of the velocity and flow rate of steam, for example, is a known problem in industrial control because steam is widely used as an energy carrier in many processes and because measurement of steam flow is a complicated task. The main reason for this complication is the presence of two fractions in the flow, a gaseous or vapour phase which is mixed with a liquid phase (water). The liquid phase moves in the pipe in the form of water droplets of various sizes, fluctuating water aggregates and water condensate which collects in the bottom of the pipe if quality of steam is low. Each of the components moves with different speed. The proportion between these components varies in time, water aggregates can combine together and water condensate can suddenly be picked up and be accelerated by the flow creating a “hummer effect.” In addition, the quality of steam changes along the pipe depending on the temperature outside of the pipe, pipe insulation, pipe bending, etc. All these factors make steam flow complicated for measurement.
A number of solutions have been proposed for measuring steam flow. Some are based on tracing the electrical properties of steam and water by measuring capacitance of the fluid at several points along the pipe or by tracking the variation of fluid density with ultrasound. The main drawback of these methods is high inconsistency with operating temperature. High-power industrial boilers run at temperatures higher than 350° C. which are beyond the limit of capacitive and ultrasonic methods. Other solutions based on gamma-irradiation methods could be applicable for steam measurement; however, gamma-irradiation is expensive and it creates a risk for operating personnel.
Cross-correlation methods for non-invasive measurement of fluid flow using optical means are known in the art. Optical methods usually are not adversely affected by high temperature because light sources and photodetectors can be located remotely from the hot measuring zones. U.S. Pat. No. 6,611,319 (Wang) describes an optical flow meter which is based on registration of the light twinkled (scintillated) due to the small changes of the refractive index with changes in temperature. The moving fluid is transilluminated by a single light source and the direct light is measured by two photodetectors spaced apart along the direction of flow. A cross-correlation function between signals from those photodetectors is calculated and a position of its maximum is determined. This position provides the average time which is necessary for the flow to move from one photodetector to the other. Consequently, the ratio of the distance between the photodetectors to the time delay gives an estimate of the average velocity of the flow.
A similar correlation technique has been described in WO 02/077578A1 (Hyde) for measuring gas flow in large pipes using attenuation of the light by the gas stream. Different constituents in the moving gas may have different absorption in the infrared region, which will cause modulation of the light passing through the pipe.
However, both scintillating method of Wang and the infrared absorption method of Hyde require long optical paths in order to accumulate enough abnormalities in the flow. Such methods require minimum pipe diameter of about one meter in order to perform reliable flow measurements. Diameters such as these are too big for steam pipelines where maximum diameter is 12 inches (30 cm) and most pipe sizes are from 2 inches (5 cm) to 6 inches (15 cm). In addition, the highly divergent light beam from the single light source used in the scintillating method of Wang spreads the time delay because different portions of the fluctuated flow cross the beam at different locations. This reduces the accuracy of the measurement. Collimated beams used in the infrared absorption method of Hyde are not affected by this effect, but steam does not absorb much light. High quality steam, in particular, is highly transparent over a wide range of wavelengths. Unscattered light, therefore, has a very low modulation depth due to the high intensity of direct light from the light source. In addition, none of the optical methods described above have been applied for sensing the quality of steam, which is of the same importance as velocity and flow measurement.
Therefore, there is a need for an apparatus and method for sensing the flow velocity of mixtures of gas and liquid such as occurs, for example, for steam moving in small pipes.